Also called ether.

Up until fairly recently (like around the turn of the century), the common consensus among physicists was that light must travel through this ultra-dense, ubiquitous material called aether. After all, light travels incredibly quickly, is a wave, and therefore needs a medium. Since it travels so quickly, the medium must be ultra dense. Since light is everywhere, the medium must be everywhere. Hence the invention of aether, an ultra-dense, omnipresent, invisible medium that light travels through. Now everyone and their brother knows that light, and, for that matter, the other members of the electro-magnetic spectrum, needs no medium for travel.